The present invention relates to a hydraulic booster or hydraulic force multiplying apparatus which may be used for the brake apparatus of a vehicle. More particularly, the present invention relates to a hydraulic booster including a booster body having a stepped bore, an operating rod operated by an input member, e.g. a brake pedal, and slidably arranged in the smaller bore of the booster body, and a power piston slidably arranged in the larger bore of the booster body and defining a boost chamber.
Such a conventional hydraulic booster includes means to throttle hydraulic oil flow, which is supplied to a power steering apparatus, to form on the upstream side of the power piston hydraulic pressure which is used to multiply force. Usually, such a hydraulic booster includes a spool axially and slidably cooperated with the power piston. When the operating rod is operated by the brake pedal to cause relative axial displacement of the power piston, the spool is displaced toward the power piston, and a throttle valve formed between the spool and the power piston operates to throttle the above-mentioned hydraulic oil flow. Hydraulic pressure produced upstream of the throttle valve is introduced in the boost chamber so that the power piston hydraulically follows the operating rod.
In the conventional hydraulic booster, the throttle valve is formed between the spool and the power piston, and its opening is determined by relative axial displacement. To this end, a conical stepped portion is formed on the outer periphery of the spool, while a shoulder portion cooperating with the conical stepped portion is formed on the inner periphery of the power piston. The hydraulic oil flow passage is formed through the throttle valve. When the spool is axially displaced relative to the power piston, the conical stepped portion approaches the shoulder portion to throttle the hydraulic oil flow, and a predetermined hydraulic pressure is produced upstream of the throttle valve.
Such a conventional throttle valve which responds to the relative axial displacement between the two members is limited to being formed between a spool and the power piston. However, by thus forming a spool valve including hydraulic passages in the power piston, the construction of the spool and the power piston becomes very complex and large, so that the hydraulic booster is also very large. The operational characteristics of the throttle valve which is formed between the axially moving spool and the power piston are also limited by the axial relative displacement and axial dimension of the spool and the power piston, so that an accurate, desired throttle effect cannot be expected.